This is an excellent 500mm tele for the price- if you can find one. I bought mine here on FM, after poking around for 6 months or se now and then. Mine is the older EX APO version, not the newer one with the DG coating, which is still a good deal against the Nikon. Don't let anyone fool you- the DG coating is better, but the older version without it is still a top quality telephoto lens, comparablerible in image quality to the Nikon or Canon 500.

With a modified Nikon 1.4x TC, it is still extremely sharp. It won't aupotfocus with even the 1.7x Nikon TC. I do not recommend the Sigma TCs because they are not as sharp. This is a bummer because, as I said, you have to actually do surgery on a Nikon TC to make it work with this lens.

The AF is fast, but not quite up to par with the Nikon AF. I have gotten excellent bird shots with this lens, but none of birds in flight yet. I have heard that it is not nearly as easy to do the BIF shots with the Sigma as opposed to the Nikkor, and I believe it. I do not know if the newer DG version has a faster AF motor or newer design, by the way.

The bokeh is great at wide apertures, but is not up to par with the newer Nikkors when stopped down past around f/3.5.

It's also 1.5 pounds lighter than the Nikon!

All in all, a great lens, and a serious alternative that save a lot of money.